Research into complaints filed with the federal government and other sources suggest the problem may not be restricted to new, third-generation 2010 Priuses and could involve vehicles dating back to the 2005 model year or even earlier.

Problems afflicting the Prius are compounding the woes of Toyota, the world's biggest carmaker, which already has recalled 8.1 million cars worldwide for unintended acceleration issues. Toyota is expected to announce a recall soon of some 300,000 Priuses, a model that is the world’s most popular hybrid and the best-selling car of all types in Japan.

After initially downplaying complaints by owners that 2010 Prius brakes could release unexpectedly, the troubled Japanese manufacturer acknowledged Thursday that it had quietly instituted a “fix” since sometime in January to solve the problem with the sedans.

But even as Toyota decides what to do next with the estimated 300,000 new-generation Priuses now in the field, an investigation by my Web site TheDetroitBureau.com finds that complaints about the hybrid’s brakes and other traction systems may extend back well before the launch of the third-generation Prius last spring.

We studied the extensive National Highway Traffic Safety Administration files for the Prius and found hundreds of customer complaints either directly detailing problems with Prius brakes prior to the 2010 model year or outlining compound issues that appear to involve both brake and accelerator issues.

That is on top of other problems, such as sudden headlight failures and unexpected powertrain shutdowns, that have been identified with the vehicle, which has become a favorite for both environmentally minded and high-tech-oriented buyers in the United States and abroad.

Observers suggest a consistency to the reported Prius issues in that most seem directly linked to the vehicle’s numerous electronic systems — a fact Apple co-founder Steve Wozniak pointed out last week when he said his car would “go wild” at times due to an apparent glitch with its cruise-control system.

While Toyota officials and safety experts have expressed frustration at the difficulty of reproducing such problems, Wozniak said he could make his Prius act up at will.

TheDetroitBureau.com first reported problems with the 2010 Prius brakes in December. Owners have complained both to Toyota and NHTSA that the brakes have an unsettling tendency to release for up to a second when the vehicle hits a pothole, bump or icy patch.

Toyota now blames the “aggressive” programming of the 2010 Prius’s anti-lock brake system, or ABS, which is designed to prevent skids.

The situation appears to be compounded by the dual-function brakes on the hybrid, which are designed to initially regenerate energy normally lost in braking and store it in the vehicle’s batteries. Under more aggressive braking, or if the vehicle senses a skid, conventional hydraulic brakes take over.

It is unclear why the company has delayed implementing a fix for the 2010 Prius sedans now on the road, but company spokesman Mike Michels says the automaker wants to make sure the changes work “in the field” as well as vehicles still on the assembly line.

The bigger question is whether Toyota is looking at problems that could extend prior to the 2010 model year.

Drivers like Jeff Zuhlke, of Wales, Wis., are convinced the problem extends to earlier models. Zuhlke says he has experienced the feeling of brake loss “many times,” in his 2005 Prius under conditions that would be unlikely to disrupt the operation of other vehicles, “such as when I’ve gone over small depressions in the road while braking. I’ve also had that same sensation whenever I come to a stop at a specific stop light that has railroad tracks near the light.”

Another Wisconsin motorist who has reported problems is Susan Yonish, who said her older Prius “had an actual crash due to the brakes not stopping the car in bumper-to-bumper traffic.”

If, as Toyota said last week, the Prius’ brakes could release for as much as a second, that would be the equivalent of driving 44 feet, or roughly three car lengths, at just 30 mph, a significant distance in heavy traffic.

“There seems to be braking issues going back” before the 2010 model-year, said Sean Kane, a well-known, if controversial, figure in safety-related litigation circles.

Like a number of critics, Kane, head of Safety Research and Strategies Inc., in Rehoboth, Mass., has questioned whether there are electronic gremlins at work, particularly with the Prius, where reports of brake problems and of unintended acceleration “may be related,” he says.

Electronic glitches do seem to be showing up in the Prius, and in more than just the latest model, based on federal documents covering a range of issues, such as intermittent headlight failures. How serious these problems are, and how they differ from other automakers’ electronic issues, remains to be seen.

Toyota officials insist that the braking problem is limited to the 2010 model year. But it’s unclear why problems would be limited to the latest model.

Though the new model is billed as the third-generation hybrid and has undergone some distinctive exterior and interior body updates, Toyota officials made a point of saying there were no significant changes to the vehicle’s distinctive Hybrid Synergy Drive system, even though the displacement of the engine was bumped up to 1.8 liters.

Spokesman Michels said the key difference is not the drive train but a recalibration to “reduce ABS over-sensitivity.” Nonetheless, Toyota has previously acknowledged that there were some issues with earlier Prius traction control technology, which also involved the programming of the hybrid’s brake controller. ABS, traction control, electronic stability control and various other technologies all rely, at least in part, on braking to improve the stopping, handling and stability of today’s cars, including Prius.

Toyota’s credibility has been stretched thin by the recent spate of problems, including the ongoing recalls over the unintended acceleration issue. When the first recall was announced in October, the automaker pointed blamed loose floor mats and denounced talk of other possible problems as “unwarranted speculation.”

But on Jan. 21 Toyota issued a new recall for millions of vehicles troubled by potentially sticky accelerators. And while the company continues to rule out electronic glitches, federal regulators are making that possibility a high priority of their ramped-up investigation.